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AMD
AMD, Master Technician
Category: Car
Satisfied Customers: 2724
Experience:  L1 Advanced Level Diagnostics Specialist
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My check engine light came on so I took it to a mechanic.

Resolved Question:

My check engine light came on so I took it to a mechanic. He did a compression test and said that piston number 3 was at 125% whereas the other three were fine. He said he needed to replace the rings on that piston. In the next breath he said that whenever you replace a piston it means you need to either get a new engine or have the present one rebuilt. He said he has a friend that sells used engines (which I wouldn't get becaue I don't trust used engine from junkyards) and he could get one for me if I liked. I asked him if everything else was fine, why did I need my entire engine rebuilt? He said its required when you replace one piston to just rebuild the engine. Is this true?
Submitted: 11 months ago.
Category: Car
Expert:  AMD replied 11 months ago.

AMD :

Hello and thank you for trusting us with your Car question, My name is XXXXX XXXXX I will be assisting you today.

AMD :

Are you available to discuss your question at time time?

AMD :

If I may ask, what make,model and year vehicle do you have?

Customer:

It's a Chrysler PT Cruiser 2008

AMD :

OK I See. How many miles does your vehicle ahve?

AMD :

Have*

Customer:

124,000

AMD :

OK I See. Was the vehicle running poorly at the time when the check engine light came on?

Customer:

it doesn't run poorly at all

Customer:

it still runs fine

AMD :

OK Do you have a safety inspection or an emissions test coming up soon that you need the check engine light to be fixed?

Customer:

no

Customer:

The mechanic did a compression test and said it was the third piston that was misfiring

AMD :

OK Let me explain something. An engine with a low compression in one cylinder would run very poorly and you can feel a misfire which is often felt as a vibration when you accelerate and extremely poor gas mileage.

AMD :

So my first concern here is the accuracy of the diagnosis, Do you trust this mechanic?

Customer:

A friend referred me to him.. I only met him today

Customer:

He said when he opened the hood he could see the engine vibrating but I don't feel it vibrating when I accelerate

AMD :

OK I See. Well rebuilding or replacing the engine is a major job and is expensive, What I highly suggest you do is to get a 2nd opinion and it would be great to get that opinion from a dealership to be sure that this mechanic didn't miss anything simple because first there are a lot of things that would cause a misfire and there are a lot of other things other than piston rings that would cause a low compression like a leaking head gasket.

Customer:

The only thing I've ever felt was on a few occasions it idled when you startup the engine

AMD :

OK I see. I suggest you get a 2nd opinion from a dealership. Let us discuss the engine options if the 2nd opinion matched the first one, Do you have any questions before we talk about the engine options?

Customer:

I'd like to talk about the engine options

Customer:

Well I did want to know if its true that replacing a piston means you have to do an engine rebuild

AMD :

OK I agree with you that the used engine option is of course not the ideal option. However let me talk about the used engine advantages and disadvantages.

Customer:

Its not an option at all. I don't trust junkyard engines

AMD :

OK I See. then let me move onto your question.

AMD :

The reason your mechanic is telling you a complete rebuild is needed is the fact that the engine is going to be removed from the vehicle, you are going to replace only one piston and one piston ring then put the engine all back together then install it in the vehicle, there's no guarantee that the other rings will not do the same thing a week later then you will have to repeat the same thing all over and the cost will be extremely high.

AMD :

So since you are going to remove the engine from the car and take it apart, might as well recondition the entire engine so now you know for a fact that you will not have any issues with the engine.

Customer:

well he did say that

AMD :

Replacing the pistons and rings require the engine to be taken apart to the bare bones.

AMD :

I highly suggest you do a complete rebuild because we all have experience and trust me you don't want to take an engine apart to replace only one part.

Customer:

he said it would take week to do this

AMD :

That is correct and here's why

Customer:

so then he is correct that a misfiring piston means you need to have the engine rebuilt

AMD :

First when the engine is out of the car, they will take everything apart and send the engine bare block and head to the machine shop to recondition them basically bringing them to a brand new engine specifications. Also the machine shop will recondition the head which is the top part of the engine. Then the mechanic in this time will be getting your parts. Engine parts are not available over the shelf and most of the time will have to be ordered.

AMD :

Once the engine is back from the machine shop which typically takes 3-5 days the mechanic will reassemble the engine outside of the vehicle then install it back into the car.

AMD :

The mechanic is correct that if you have low compression a complete engine rebuild is the best option, However you'd want to get a second opinion to make sure their diagnosis is correct before you make the investment of rebuilding the engine.

Customer:

What else could it be?

AMD :

Also you do want to shop around for the engine rebuild prices. Get a quote from multiple mechanics and compare prices and warranties.

AMD :

If you have low compression, it could also be a leaking head gasket which is a much less major job and doesn't require a full rebuild it only requires the head gasket to be replaced. still a big job but not even close to what a full engine rebuild would cost and typically a head gasket job would take a day or two.

AMD :

I suggest you stop by your local Chrysler dealership and have them give you a 2nd opinion by looking at the car.

AMD :

It will cost you a diagnosis fee but if the original mechanic made a wrong diagnosis then you have automatically saved a lot of money

AMD :

It's very important that the diagnosis be double checked when it comes to a major job like this.

Customer:

The dealerships always charge such high fees for everything

Customer:

But I think you are right better to go the people that manufacture the car for the diagnosis

AMD :

I know, Typically the dealership will charge around 100-150 dollars for diagnosis. But you just need their diagnosis, also have them give you an estimate. Of course they are the most knowledgeable people in your car and I would trust their diagnosis. Also they would give you the highest quality job, so if their price is a bit more than your mechanic but not a lot more I would go with the dealership because they do the best job because they are the specialists in your car.

AMD :

Did you have any other questions for me today?

Customer:

Does the fact that the car has a rough idle (occasionally) at start-up confirm that the first mechanic's diagnosis may be correct?

AMD :

It may be correct given that the car has a rough idle, but if they did something wrong during the compression test and their reading is wrong then it could be a bad injector or a bad coil that would also cause a misfire which is nothing even close to being major like a piston\ring or head gasket. that's why a second opinion is definitely a good idea

Customer:

Would a bad injector or bad coil cause a rough idle?

AMD :

It would cause a rough idle for sure and would cause the check engine light to indicate a misfiring cylinder.

AMD :

Because every cylinder needs fuel from the injector and fire from the spark plug\ignition coil and compression from the piston\ring to run right

AMD :

if any of the three has an issue it will misfire and cause the rough idle and lack of power and the check engine light

Customer:

Also I notice that when I'm stopped at a stop sign or red light the check engine light starts flashing (when its normally steady). why does stopping for a few minutes cause the check engine light to flash?

AMD :

When the check engine light starts to flash this indicates that the engine is misfiring right now. It's just a way to warn the driver that the engine is not running right. if you continue to drive the vehicle like this for a long time like 6 months to a year the catalytic converter which is a part of the exhaust and is a very expensive part of the exhaust will get ruined and also will cause the check engine light to be on even after you fix the current misfire issue.

Customer:

I just thought of something: wouldn't the compression test the mechanic performed have shown if it was a bad coil or bad injector?

AMD :

Typically a misfiring engine recovers from a misfire at higher engine speeds and is more prone to misfire hard under heavy load or at idle when the speed is the slowest.

AMD :

The compression test is the 3rd thing any mechanic would do in the misfire diagnosis because that is the most involved test out there. Typically we start by a method called swapping coils and swapping injectors. For example if we have a bad coil or injector in cylinder 3, if you change this injector or coil with cylinder no.2 then start the car, cylinder no.2 would start running wrong not 3. so this is a very quick way to know if you have a bad coil or a bad injector. If you swap both and it still has number 3 misfiring then we do a compression test which is step 3 after swapping an injector and a coil. if the compression is low then we perform a test called a cylinder leak down test which tells us where is the leak coming from that is causing a low compression number.

AMD :

With the cylinder leak down test we know if it's a head gasket that is leaking or a piston ring or sometimes even a bad valve.

AMD :

So all these steps has to be done in order to accurately diagnose this issue. If your mechanic came back to you within 1/2 hour telling you the car needs an engine, I can tell you with confidence that they are incompetent and dishonest.

Customer:

He went straight to the compression test

Customer:

And called me over so I can see the temperature gauge on the third piston was at 125

AMD :

Well that's the first mistake. You never skip steps in a diagnosis process, because a wrong diagnosis could cost you a lot of money and that is just not right. before you invest all this money you definitely want to stop by the dealership and have them take a look to make sure that the first diagnosis is right.

AMD :

So to summarize I cannot emphasize how important it is that you get a second opinion on this matter before you do anything

Customer:

I will definitely go to the dealership and get a second opinion. I will not tell them what the first mechanic said... I will just tell them my check engine light is on and see what they say

AMD :

Exactly. I would trust what the dealership says more than any other mechanic. Also compare the prices from both the first mechanic and the dealership. Sometimes the price is only a little bit higher on the dealership and is worth spending a bit more.

AMD :

Did you have any other questions for me today?

Customer:

Does an engine rebuild make the car like new despite having 125,000 miles

AMD :

Of course. Regardless of the mileage, once an engine is rebuilt it is basically as good as new because everything vital inside of it is replaced with a new part and\or machined to like new specification.

Customer:

wow so if you decided to get a new engine its cheaper than buying a new car?

AMD :

Well the car has a lot more components than the engine. You still have the transmission, the brakes the suspension and a lot of various components that are still with 125,000 miles but having a new engine is a very big reliability boost at 125,000 miles and would give you the piece of mind that you won't have any engine issues for another 125,000 or even possibly more.

Customer:

very last question of the night.... do you trust the cars at auctions.. not the auctions that are open to the public but the ones open only to dealers

AMD :

I personally have dealt with cars that are open to the dealerships only and I would tell you that I wouldn't take a car from an auction even if it was given to me for free :-) Typically they only give you a few minutes to check a car and even if you go a day before the auction to check the cars you won't know how the car is. Sure you will buy it cheaper but you won't know how good the car is until it's too late and you've already bought it. These days it's rare to buy a good car from the auction. If you are a mechanic then it's OK because you are buying a car to fix, but being a non mechanic it can get expensive very quick and whatever you save buying a car cheaper from the dealer auction you will end up spending fixing the car. I would just buy either private on craigslist or buy from a reputable dealership.

Customer:

Thanks for your advice.. I much appreciate it.

AMD :

You're welcome! I'm glad I could help.

AMD :

A positive rating for my input would be greatly appreciated.

AMD :

Thank you for choosing justanswer.com

AMD, Master Technician
Category: Car
Satisfied Customers: 2724
Experience: L1 Advanced Level Diagnostics Specialist
AMD and 5 other Car Specialists are ready to help you

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